What is the difference between a direct serial cable and a null modem serial cable?

The serial cable can be divided into straight chain and intersection. Generally for straight line extension PC equip, will 2, 3, 5 connect 2, 3, 5, because PC universal male. But the devices are mostly female, so they are just plain, and it can also be used to extend the connection; the crossover PC is connected to the PC with 2 pairs of 3, 3 pairs of 2, 5 to 5, and the general two are female at both ends!

Before the computer appeared, the EIA established the RS232 standard for connecting serial devices.

After the emergence of PC, the existing serial devices become PC peripherals. Natural use of the RS232 standard. Now the serial communication interface of PC is adopted EIA-RS-232C standard. C represents the latest change in 1969. The EIA-RS-232C standard provides for electrical characteristics, logic and various signal line functions. The EIA-RS-232C standard uses the positive and negative voltage to represent the logic state, on the data signal line if the voltage between the -3v~-15v represents the logic 1, if the voltage between the +3v~+15v represents the logic 0, in the control signal line if the voltage between -3v ~-15v indicates the disconnection state. If the voltage between +3v ~+15v indicates the on state.

Voltages between -3v ~+3v and below -15v, above +15v, are meaningless.

The CPU and the terminal are used TTL level and the positive logic, TTL level with the 1,0v of the logic is represented by the logic 0. They are incompatible with the level and negative logic used by the EIA. Need to be converted in the interface circuit.

The EIA-RS-232C standard does not define the physical characteristics of the connector. As a result, DB-25, DB-15, and DB-9 connectors of all types are present. the COM1 and COM2 serial interfaces of the PC are made of DB-9 connectors .

The ELA-RS-232C standard stipulates that when the BER is less than 4%, the wire length 15m is agreed. In practical applications. When using 9600b/s, ordinary twisted shield wire, transmission distance of up to 30m ~35m.

The PC's COM1 and COM2 two serial interface DB-9 connectors are male (PIN) headers that provide 9 pin functions for asynchronous communication . are: ① foot (DCD) data carrier detection, ② feet (RXD) receive data. ③ Pin (TXD) sends data. ④ foot (DTR) data Terminal ready, ⑤ foot (SG) signal ground, ⑥ foot (DSR) data device ready. ⑦-pin (RaS) request sent, ⑧ (CTS) clear send. ⑨ foot (RI) ringing indication. DB-9 male (PIN) head arrangement order for example with left, DB-9 female (hole) head in order such as right, pay special attention to the male pin number from sitting to right, the female head is from right to left . eia-rs-232c Standard, there are three send signals: TXD, RTS and DTR. The typical output current for each line is ±8ma/±12v.

Usually powered by RTS and DTR. Approximately 192mW of power is available.


The PC side is generally male,

Combine the instructions above. Assume that the modem is not involved. The serial port data transmission only must have the receiving data end ② foot and sends the data end ③ the foot to be possible. The receiving data end of a device should be connected to the sending data end of a device. That is, the reception and transmission intersect each other, the signal to the corresponding phase.

Question 1: When to use crossover. When to use Direct Connect?
Standard serial Port pin 2 is RX, pin 3 is TX, (that is, the nine-pin serial port 2nd pin is connected to the RXD pin of the master chip, the 3rd PIN is connected to the main control chip TXD pin), it is assumed that the SCM Development Board serial port and the computer serial port are standard serial port. The two serial ports should be connected with a crossover line. But in real life straight line most (may be straight wire production up without cross-wire trouble it), in order to cooperate with the use of direct connection, in the artboard, the main control chip RXD pin connected to the serial port of 3 feet. The TXD pin of the master chip is connected to the 2 pin of the serial port, so that the serial port on the Development Board is not the standard serial port, that is, 2 becomes tx,3 Rx, and the standard serial connection should use straight line. Look at a development board on the serial port is the standard serial port is also a non-standard serial port, it is essential to see how the schematic is connected!

Overall: RS232 's DB-9 interface has three types of connections: Male to female, male to male. Mother to mother. Each of the three connectors has crossover lines and straight lines, so there is a total of 6 connections in the same way.

Question 2: How to identify the cross-serial line and direct-connect serial line?
Check it with a multimeter. Suppose 2 and 2 pass. The general 3 and 3 are straight serial lines. Assume that the 2 and 3 pass, 3 and 2 general are cross-serial lines.
Someone confused the outside of the mother is an insulating rubber, multimeter probe can not be inserted how to measure? Then cut a small section of solder solder (I have also considered using wire, but the wire is too thin.) We cannot guarantee that it will be in perfect contact with the metal part of the mother's mouth. The solder wire is thicker, it can be very good contact with the metal inside the mother's mouth. Then touch the multimeter probe to solder wire to test it.
Of course, assuming that every time you get a serial cable to take a multimeter to measure whether it is straight or cross is not very troublesome, teach you a easy way to distinguish the good method, that is: Two mother is cross, a male and female direct connection .

(second, the male serial line seems to be very small)

Note: It is strongly recommended not to live pluggable serial, at least one end is off, when the call waits, otherwise, the serial is fragile.

What is the difference between a direct serial cable and a null modem serial cable?